Instruments for measuring electrocardiogram (ECG) signals often detect electrical interference corresponding to a line, or mains, frequency. Line frequencies in most countries, though nominally set at 50 Hz or 60 Hz, may vary by several percent from these nominal values.
Various methods for removing electrical interference from ECG signals are known in the art. Several of these methods make use of one or more low-pass or notch filters. For example, U.S. Pat. No. 4,887,609, whose disclosure is incorporated herein by reference, describes a system for variable filtering of noise in ECG signals. The system has a plurality of low pass filters including one filter with a 3 dB point at approximately 50 Hz and a second low pass filter with a 3 dB point at approximately 5 Hz.
U.S. Pat. No. 6,807,443, whose disclosure is incorporated herein by reference, describes a system for rejecting the line frequency component of an ECG signal by passing the signal through two serially linked notch filters. U.S. Pat. No. 5,188,117, whose disclosure is incorporated herein by reference, describes a system with a notch filter that may have either or both low-pass and high-pass coefficients for removing line frequency components from an ECG signal. The system also provides means for removing burst noise and for calculating a heart rate from the notch filter output.
U.S. Pat. No. 5,564,428, whose disclosure is incorporated herein by reference, describes a system with several units for removing interference: a mean value unit to generate an average signal over several cardiac cycles, a subtracting unit to subtract the average signal from the input signal to generate a residual signal, a filter unit to provide a filtered signal from the residual signal, and an addition unit to add the filtered signal to the average signal.
U.S. Pat. No. 5,349,352, whose disclosure is incorporated herein by reference, describes an analog-to-digital (A/D) converter that provides noise rejection by synchronizing a clock of the converter with a phase locked loop set to the line frequency.